The present invention relates generally to the oxidation of a broad spectrum of waste material and, more particularly, to a method and apparatus for the wet oxidation of organic waste, including organic waste having inorganic contaminants such as metals or metallic compounds, at supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical, pressures.
One known method of oxidizing waste material, such as municipal sewage or industrial sludges, commonly referred to a supercritical water oxidation, is disclosed in U.S. Pat. Nos. 4,113,446; 4,338,199; and 4,543,190. In this oxidation method, the waste material is pressurized and fed into a reactor vessel along with compressed oxidant, either oxygen, air or oxygen-enriched air, and maintained therein under supercritical temperature conditions, that is at a temperature above 374.degree. C., and, elevated pressure, to cause rapid oxidation. Under supercritical conditions, water exists in a high-energy, dense-vapor form and is consequently capable of dissolving normally insoluble organics thereby permitting separation from and disposal of inorganic contaminants such as metallic halides, sulfur compounds, light metal elements such as sodium and the like, and heavier toxic metals such as lead, mercury and the like. The end products of the supercritical water oxidation process are carbon dioxide, salt, water and heat.
A commercially practiced method of carrying out supercritical water oxidation, and a reactor vessel particularly adapted therefor, are disclosed in U.S. Pat. No. 4,822,497. As disclosed therein, an aqueous waste stream containing organic and inorganic material is pressurized and fed along with compressed oxygen or air into the upper region of a pressure vessel. This reactor vessel is comprised of two zones: a supercritical temperature zone and a subcritical temperature zone, the subcritical temperature zone being maintained in the lowermost region of the reactor vessel and the supercritical zone being maintained above the subcritical zone. The pressurized feed waste material and the compressed oxidant are admitted via a feed pipe extending into the upper region of the reactor vessel into the supercritical temperature zone. Alkaline material may be injected into the feed stream in order to neutralize any acids formed during the oxidation process. Reaction product gas and effluent from the supercritical temperature zone exit via a nozzle and associated piping. A portion of the effluent is recycled to heat the incoming aqueous waste feed stream, while the remainder of the reactor effluent is cooled, depressurized and discharged in separate gaseous and liquid product streams.
Oxidation of the organics and oxidizable inorganics takes place in the supercritical temperature zone. The material, usually inorganic salt, that is initially present or formed in the supercritical or semicritical fluid phase and is insoluble therein forms dense brine droplets or solid precipitates which inertially impinge on, and fall by gravity into, a liquid phase, still at elevated pressure, provided in the lower temperature, subcritical zone maintained in the lower portion of the vessel. The liquid phase in the subcritical zone provides an aqueous medium for trapping solids, for dissolving soluble materials, and for forming a slurry of insoluble materials. The resultant solution or slurry is removed from the reactor vessel via a pipe opening to the lower region of the vessel.
The inorganic salts which are insoluble in the supercritical or semicritical fluid phase in the reactor vessel may to a lesser or greater extent be tacky, possibly depending on the nature of the feed material and/or operating conditions in the reactor. Organic halogen and sulfur in the waste feed material have been found to react with alkaline material in the feed to form inorganic salts which have extremely limited solubility at supercritical conditions. These insoluble inorganic salts have been known to deposit on the walls of the reactor vessel with the supercritical zone and also on parts of the outlet nozzle from the supercritical zone. Uncontrolled deposition and build up of solids on the reactor vessel wall will require periodic shut down of the reactor to permit flushing and backwashing to remove accumulated solids. As the frequency at which shut down will be necessary will depend on the "tackiness" of the solids, the operating conditions within the reactor vessel, the nature of the waste feed material and likely other conditions, shut downs cannot be predictably planned.